1. Field of the Invention
The present invention relates to a vibration damping device adapted for installation between components which make up a vibration transmission system, to provide vibration-damped coupling and/or vibration-damped support between the components making up the vibration transmission system; and relates in particular to a fluid filled type vibration damping device which utilizes vibration damping action based on the flow effect of a non-compressible fluid filling the interior.
2. Description of the Related Art
There are a number of vibration damping devices known in the art for the purpose of installation between vibration-damped components, such as the power unit and the vehicle body of an automobile for example, in order to provide vibration-damping coupling and/or vibration-damping support between these vibration-damping components. One device of this kind is a fluid filled type vibration damping device the interior of which is filled with non-compressible fluid and which utilizes vibration damping action based on the flow effect of the sealed fluid. One such fluid filled type vibration damping device has a structure in which, for example, a first mounting member is positioned spaced apart from the opening at one axial end of a second mounting member of cylindrical shape, with the first mounting member and the second mounting member elastically coupled together by a main rubber elastic body and with a flexible film disposed so as to cover the opening at the other axial end of the second mounting member, thereby forming in a region to the inner peripheral side of the second mounting member between the opposed faces of the main rubber elastic body and the flexible film a fluid chamber which is isolated from the outside space and which is filled with a non-compressible fluid. A partition member supported by the second mounting member is disposed within the fluid chamber, thereby forming to one side of the partition member in the axial direction a pressure receiving chamber a portion of whose wall is constituted by the main rubber elastic body, and forming to the other side of the partition member in the axial direction an equilibrium chamber a portion of whose wall is constituted by the flexible film. An orifice passage connecting the pressure receiving chamber and the equilibrium chamber together is formed in the partition member.
In this type of fluid filled type vibration damping device, when a load targeted for vibration damping acts across the first mounting member and the second mounting member, internal pressure fluctuations arise in the pressure receiving chamber a portion of whose wall is constituted by the main rubber elastic body, thus giving rise to a pressure differential relative to the equilibrium chamber which readily changes in volume and maintains generally unchanging internal pressure. Fluid flow through the orifice passage between the two chambers is created thereby, and vibration damping action is produced on the basis of the flow effect, e.g. the resonance effect, of the fluid.
A problem with sealed fluid type vibration damping devices of this kind is that noise and vibration sometimes occur when an impulsive load is input across the first mounting member and the second mounting member. Such noise and vibration are attributed to the phenomenon of cavitation due to negative pressure within the pressure receiving chamber in association with input of a large load. Since the problem has an adverse effect on quiet and ride comfort in the passenger cabin, various methods have been sought for resolving the problem.
As a means of solving this problem of cavitation noise, it has been proposed for example to provide a shunt hole through which the pressure receiving chamber and the equilibrium chamber communicate with each other. Specifically, since problematic noise and vibration are created by a drop in pressure within the pressure receiving chamber, by providing a shunt hole through which the pressure receiving chamber and the equilibrium chamber communicate via a shorter passage length than the orifice passage, even in the event that flow of fluid through the orifice passage cannot keep up, fluid will nevertheless be transported from the pressure receiving chamber to the equilibrium chamber through the shunt hole. Negative pressure within the pressure receiving chamber can be rapidly dispelled thereby, and noise and vibration attributed to such negative pressure can be prevented.
However, in a fluid filled type vibration damping device provided with such a shunt hole, there is a risk of a drop in vibration damping capability during input of vibration to be damped. Specifically, since the shunt hole is formed so that it is always open, fluid will be induced to flow through the shunt hole between the pressure receiving chamber and the equilibrium chamber even at times of input of vibration to be damped. Thus, there is a danger that sufficient flow of fluid through the orifice passage will not be achieved, resulting in an appreciable decline in the vibration damping capability of the orifice passage.
To address this problem, there has been proposed for example in US-2007/0075470A1 a fluid filled type vibration damping device having a shunt passage through which the pressure receiving chamber and the equilibrium chamber can communicate with each other via a shorter passage length than the orifice passage, and which is able to switch the opening at the pressure receiving chamber end of the shunt passage between the communicating state and the blocked state by a valve body which is constituted utilizing the main rubber elastic body. With this design, when a large impulsive load that could cause a cavitation problem is input, the valve body will be induced to open so that the shunt passage assumes the communicating state and allows fluid to flow through the shunt passage between the two chambers so that negative pressure within the pressure receiving chamber will be quickly dispelled. On the other hand, when vibration to be damped is input, the shunt passage will be closed off by the valve body, thereby preventing liquid pressure within the pressure receiving chamber from escaping to the equilibrium chamber through the shunt passage, so that fluid flow through the orifice passage can be effectively created and cavitation noise can be reduced or eliminated.
However, in a fluid filled vibration damping device of a structure equipped with a valve body, which is provided for opening and closing the shunt passage such as taught in US-2007/0075470A1, the durability of the valve body will tend to be a problem with repeated operation over an extended period. Furthermore, in some instances, it is exceptionally difficult to tune the operation of the valve body in such a way that it can switch the shunt passage between the communicating state and the blocked state with a high degree of accuracy in response to input of specific vibration. Also, where a special valve body is provided, the problems of an increased number of parts and more complex structure may result. Accordingly, there exists a need for a fluid filled type vibration damping device which affords both effective vibration damping action against vibration to be damped, and reduction or elimination of cavitation noise, through a simple structure with a minimal number of parts.